1. Field of the Invention
The present invention relates to coating compositions based on non-aqueous dispersions of solid fluoropolymer particles in an organic solution of acrylic polymers.
The coatings, referred to as “organosols,” may be clear or pigmented (opaque) coatings, which may be applied to a variety of substrates. In particular, the present invention is directed to coating compositions that have a high solids content and relatively few ingredients. The coating compositions are easily manufactured and may be applied using a number of art recognized techniques.
2. Description of the Prior Art
Fluoropolymer dispersion coatings are known in the art and are disclosed in, for example, Canadian Patent No. 756,165 to Koblitz et al., U.S. Pat. No. 4,314,004 to Stoneberg, and European Patent No. EP 0 960 918 to Lin et al.
Fluoropolymer dispersion coatings are known to exhibit outdoor durability, chemical resistance, and acceptable mechanical properties. The performance features of fluoropolymer dispersion coatings have led to their extensive use, for example, in the exterior building panel market. Fluoropolymer dispersion coatings are typically applied by spray and roll coating or coil coating of flat sheet stock techniques. The coating film is formed by thermal fusion of the fluoropolymer particles in admixture with an acrylic resin.
Historically, fluoropolymer dispersion coatings exhibit a relatively high viscosity at relatively low volume solids content. Consequently, as much as 65percent organic solvent by volume may be required to reduce the viscosity in order to facilitate application of the fluoropolymer dispersion coatings to a substrate.
The high level of volatile organic compounds (VOCs) of fluoropolymer dispersion coatings generally requires that the solvent vapors emitted by the wet film be captured and conveyed to a gas-fired incinerator or thermal oxidizer to destroy the VOCs. For example, the large amount of VOCs produced by coil coating flat metal sheet stock can limit the line speed of the coating application, or result in blistering of the film at higher film thicknesses. The incineration of the VOCs can also produce higher amounts of nitrogen oxide pollutants, particularly for fossil fuel-fired combustion processes.
U.S. Pat. No. 6,017,639 to Higginbotham et al. discloses a high solids thermoset fluorocarbon coating. However, the composition disclosed by Higginbotham et al. relies on expensive fluorinated surfactants or “hyperdispersants” as an essential element of the coating. The coating composition is relatively complex and includes a relatively long list of ingredients.
International Application No. WO 01/00739 to Zupancic et al. discloses a high solids thermoset fluorocarbon coating, which includes a “cross-linkable” acrylic resin. An additional required ingredient in the coating composition of Zupancic et al. is a cross-linking agent for the acrylic resin. The resulting film, although flexible and solvent resistant, provides only a modest degree of film hardness, as exemplified by the reported pencil rating of “F.”
There is a clear and well defined need for a higher solids, lower VOC content liquid fluoropolymer coating composition, which would allow for higher line speed application, reduced blistering tendency of the applied coating, and a reduced impact on the environment from VOCs. Furthermore, it would be considered by those skilled in the art to be an advance and particularly economically advantageous development if such a coating composition could be achieved by the blending of relatively few ingredients while providing a simple formulation that would be easily manufactured and readily reproducible.